Digital printing apparatus using liquid developer dispersion—also known as liquid toner—are known from US patent application publication no. 2011/0249990. The known digital printing apparatus comprises a feed roller, a developer roller, developer roller cleaning means, and an image carrying roller; the feed roller being arranged to transfer a quantity of liquid toner from a reservoir onto the developer roller; and the developer roller being arranged to transfer a portion of the quantity of liquid toner onto the image carrying roller in accordance with a charge pattern sustained on a surface of said image carrying roller. A liquid toner residue, also referred to as an excess liquid developer dispersion, remains on (the surface of) the developer roller after the imagewise transfer of the liquid toner from the developer roller to a further roller, particularly the image carrying roller.
In digital printing systems of this kind, it is necessary to remove the liquid toner residue that remains on the surface of the developer roller after contact with the image carrying roller. Any liquid toner residue that remains on the surface of the image carrying roller after contact with a transfer roller of after contact with a substrate needs to be removed as well. Conventionally, use is made of a removal device, such as a scraper, as is disclosed in US2011/0076052A1, paragraph [0044].
It has been observed in preliminary investigations leading to the invention that these highly concentrated and therefore highly viscous compacted toners are not easily decompacted and removed from rollers. Thus, the removal of such a residue can be quite challenging.
Particularly, marking particles in the liquid developer dispersion tend to form lumps in the dispersion resulting in a liquid with a non-uniform distribution of marking particles. This is called caking and often results in an increase of the viscosity of the liquid dispersion. This viscosity increase is significant and could be a tenfold increase or even more.
Liquid developer dispersion that shows caking cannot be used for printing as such and needs to be treated first in order to re-obtain a homogeneously dispersed liquid toner which has similar conductivity and viscosity properties as the starting liquid developer dispersion.
It is thought that caking is the result of marking particles that come so close into each other's neighborhood on the developer roller, so that they start to feel each other's presence and start interacting with each other. Caking can also be the result of injecting charge and applying high shearing forces which are typically present when a thin layer of liquid developer dispersion passes through a very narrow gap between two (rotating) members of the printing apparatus or huge (microsized) mechanical interaction like a cleaning blade scraping on a rotating surface.
The removal of the liquid toner residue starts then to be problematic. As a result, liquid toner residue could remain on the developer roller, which constitutes a contamination and may lead to a non-uniform distribution of fresh developer dispersion resulting in a ghost image and or image quality that is not perfect, in other words incorrect. Specific examples of issues are density instability and incorrect reproduction of fine lines. Removal of the toner residue by a removal device may reduce the issue, but is not known to solve the issue completely. It is therefore a major problem to solve the caking issue.
Unpublished European patent application no. 12 175 762.9 in the name of the Applicant describes the use of an oscillating electric field arranged to substantially decompact the chargeable imaging particles in a liquid toner residue on a developer roller, prior to or during its mechanical removal. Unpublished European patent application no 131625774 in the name of the Applicant describes the use of a loosening member with a rubbing portion, which is arranged to rub the liquid toner residue to be loosened. In use, the rubbing portion is suitably compressed by a development member, particularly the development member. This compression results in absorption of the liquid toner residue in the loosening member, and particularly the rubbing portion thereof. Thereto, the rubbing portion suitably comprises a porous material, such as an elastic foam material.
Unpublished Dutch application NL2010581 in the name of the Applicant describes the addition of an amount of dispersing agent to the excess liquid developer dispersion. This dispersing agent turns out to reduce the viscosity of the excess liquid developer dispersion, and helps to deagglomerate marking particles in the dispersion and/or decrease adhesion of the marking particles to a development member. Therewith the excess liquid developer dispersion is stabilised and its effective viscosity decreased. The amount of the dispersing agent to be added may be controlled on the basis of image information, i.e. when the transferred image is large, less toner residue remains on the development member than when the image is comparatively small. Hence, with a large image, less dispersing agent needs to be added. The dispersing agent is suitably added via a loosening member.
It is desired to improve the treatment process further, so as to facilitate the removal of excess liquid developer dispersion from the development member, in a form that does not harm the recycling of the developer dispersion.